The present invention relates to an image encoder and a method thereof in which an input image is compressed and encoded by wavelet transformation, quantization, and entropy encoding to generate an encoded code stream, and an image decoder and a method thereof in which the generated code stream is decoded to reconstruct the input image, like the JPEG 2000 scheme.
One of conventional typical image compression schemes is the JPEG (Joint Photographic Experts Group) scheme standardized by the ISO (International Standards Organization). This scheme utilizes discrete cosine transformation (DCT). When a relatively high bit number is assigned, excellent encoded and decoded images are obtained, as is known. However, if the encoding bit number is lowered to a certain degree or much lower, block deformation inherent to the DCT appears so conspicuously that subjective deterioration of image quality becomes conspicuous.
Meanwhile, recent developments have been becoming active about a scheme in which an image is divided into plural bands by a filter combining high-pass and low-pass filters, called a filter bank, and encoding is performed for every band. In those developments, wavelet transformation encoding has been considered as new prevailing technology which will take over the DCT because the wavelet transformation encoding does not have the drawback of block deformation appearing conspicuously at a high compression rate like in the DCT.
The JPEG 2000 scheme has been completely internationally standardized since January 2001, and employs a system which combines the wavelet transformation, quantization and highly efficient entropy encoding (bit modeling and arithmetic encoding in units of bit planes.) Compared with earlier JPEG schemes, the encoding efficiency has been improved actually (see Japanese Patent Application Laid-Open Publication No. 2002-27403.) These international standards define specifications concerning decoders but allows free design concerning encoders.
However, the quantization means in the JPEG 2000 scheme aims primarily at a high compression rate. There hence has been a problem that excellent performance cannot always be achieved in case where a high-quality image is required at a low compression rate.